Measles, one of the most communicable of all infectious diseases, is spiking in the United States, with three times as many cases as usual this year, the Centers for Disease Control and Prevention said Thursday. The spike is due to both foreign importations — infected travelers coming from places where measles is not under control — and local vulnerability: unvaccinated children and adults in the United States.

In a press briefing, the CDC’s director Dr. Thomas Frieden said that from January to November, there were 175 known cases of measles in the US, with 20 of those people having to be hospitalized. The agency would expect to see about 60 cases, he said. Those cases came from 52 separate travelers. Most of the time, the imported virus found only a few people to infect — but nine times, the imports caused large outbreaks, always in people who had not received the vaccine.

“It is not a failure of the vaccine,” Frieden said. “It’s a failure to vaccinate. Around 90 percent of the people who have had measles in this country were not vaccinated either because they refused, or were not vaccinated on time.” If 175 cases doesn’t sound like much, consider measles’ impact. It isn’t just an itchy rash; it can cause deafness and encephalitis, and miscarriage in pregnant women. Before the measles vaccine was achieved 50 years ago, the disease killed 2.6 million people around the world every year. Its cost to society is huge. A single importation of measles into Arizona in 2008, via an unvaccinated, infected Swiss tourist, caused a 14-person outbreak; compelled the Arizona Department of Health to track down and interview 8,321 people; caused seven Tucson hospitals to furlough staff members for a combined 15,120 work-hours; and forced two hospitals to spend $799,136 to contain the disease.

The story of that outbreak illuminates two important things about measles now. First, importations of the disease are increasing. Look at the trend line in this graphic published yesterday by CDC staff in the journal JAMA Pediatrics. That uptick on the right is entirely due to importations followed by local transmission:

The other important thing to consider is where the importations are coming from. We tend to think of diseases that cross our borders as something that originates in the developing world — but this time, it’s the industrialized world that is partly to blame. Dr. Samuel Katz, co-creator of the measles vaccine, spoke at the CDC’s press briefing yesterday. He said:

We’re not talking about measles being imported from Bangladesh and from India and from the resource-poor countries. Western Europe has had 25,000 cases of measles every year for the last three years… in great part due to vaccine hesitancy.

The greatest threat to the US vaccination program may now come from parents’ hesitancy to vaccinate their children. Although this so-called vaccine hesitancy has not become as widespread in the United States as it appears to have become in Europe, it is increasing. Many measles outbreaks can be traced to people refusing to be vaccinated … Even greater risk may come from parents who delay vaccinations rather than refusing them outright because a delayed vaccination may add more person-years of susceptibility than that due to refusing vaccination.

I wasn’t at the CDC’s briefing yesterday — I’m traveling, and listened in — but I imagine that being in the room must have been quite poignant. The measles vaccine is one of the triumphs of public health; Katz and his co-creators are believed to have saved the lives of 30 million children. Over 50 years, measles has been chased entirely out of the Western Hemisphere. Yet keeping it from becoming re-established, and eliminating it from the rest of the world, requires increasing vaccination at a time when so many are turning away. As Katz said yesterday — and perhaps it was my imagination, or a bad connection, but I thought I heard a touch of wistfulness:

It’s really nice to be worrying about 175 cases of measles. It’s a mark of progress. But it also shows how much further we have to go.